Hot carrier programming in NAND flash

ABSTRACT

A memory device includes a plurality of memory cells arranged in series in the semiconductor body, such as a NAND string, having a plurality of word lines. A selected memory cell is programmed by hot carrier injection using a boosted channel potential to establish the heating field. Boosted channel hot carrier injection can be based on blocking flow of carriers between a first side of a selected cell and a second side of the selected cell in the NAND string, boosting by capacitive coupling the first semiconductor body region to a boosted voltage level, biasing the second semiconductor body region to a reference voltage level, applying a program potential greater than a hot carrier injection barrier level to the selected cell and enabling flow of carriers from the second semiconductor body region to the selected cell to cause generation of hot carriers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to flash memory technology, and moreparticularly to flash memory suitable for low voltage program and erasein a NAND configuration.

2. Description of Related Art

Flash memory is a class of non-volatile integrated circuit memorytechnology. Traditional flash memory employs floating gate memory cells.As the density increases in memory devices, and the floating gate memorycells get closer and closer together, interference between the chargestored in adjacent floating gates becomes a problem. This is limitingthe ability to increase the density of flash memory based on floatinggate memory cells. Another type of memory cell used for flash memory canbe referred to as a charge trapping memory cell, which uses a dielectriccharge trapping layer in place of the floating gate. Charge trappingmemory cells use dielectric charge trapping material that does not causecell-to-cell interference like that encountered with floating gatetechnology, and is expected to be applied for higher density flashmemory.

The typical flash memory cell consists of a field effect transistor FETstructure having a source and drain separated by a channel, and a gateseparated from the channel by a charge storage structure including atunnel dielectric layer, the charge storage layer (floating gate ordielectric), and a blocking dielectric layer. According to the earlyconventional charge trapping memory designs referred to as SONOSdevices, the source, drain and channel are formed in a silicon substrate(S), the tunnel dielectric layer is formed of silicon oxide (O), thecharge storage layer is formed of silicon nitride (N), the blockingdielectric layer is formed of silicon oxide (O), and the gate comprisespolysilicon (S).

Flash memory devices generally are implemented using NAND or NORarchitectures, although others are known, including AND architectures.The NAND architecture is popular for its high density and high speedwhen applied to data storage applications. The NOR architecture isbetter suited to other applications, such as code storage, where randombyte access is important. In a NAND architecture, the programmingprocesses typically rely on Fowler-Nordheim (FN) tunneling, and requirehigh voltages, such as on the order of 20 volts, and require highvoltage transistors to handle them. The addition of high voltagetransistors on integrated circuits, in combination with transistors usedfor logic and other data flow, introduces complexity in themanufacturing processes. This increased complexity in turn increases thecosts of the devices.

Accordingly, it is desirable to provide a new memory technology suitablefor low voltage programming operations, and which is configurable in aNAND architecture.

SUMMARY OF THE INVENTION

A memory device is described that includes a plurality of memory cellsarranged in series in the semiconductor body, such as can be applied fora NAND string in a NAND array, having a plurality of word lines coupledto corresponding memory cells. Control circuitry is coupled to theplurality of word lines and to the semiconductor body adapted forprogramming a selected memory cell by hot carrier injection using aboosted channel potential to establish the heating field across thechannel of the selected memory cell. Hot carrier injection using thisprocess can be implemented by control circuitry which applies a passvoltage to word lines on a first side of the selected word line during aprogram interval to boost a first semiconductor body region bycapacitive coupling to a boosted voltage level, which applies a programvoltage to the selected word line during the program interval, and whichapplies a biasing condition to a second semiconductor body region on asecond side of the selected word line during the program interval toestablish a reference voltage level. A switching voltage is applied to aword line, or switching voltages are applied to more than one wordlines, adjacent the selected word line. The switching voltage has afirst stage during which the first and second semiconductor body regionsare isolated by turning off the corresponding cell, while the boostedvoltage level and reference voltage level are established in the firstand second semiconductor body region. The switching voltage has a secondstage in which the corresponding cell is turned on, to couple thereference voltage level to the selected memory cell and cause hotcarrier injection.

The selected word line is biased during the program interval by aprogram voltage which is sufficient to overcome the channel hot carrierinjection barrier level. However, this program voltage can besignificantly lower than is required for typical Fowler Nordheimprogramming. The other word lines corresponding with the plurality ofmemory cells receive a pass voltage which is lower than the programvoltage in order to inhibit disturbance of the other cells. Theswitching voltage during the second stage of the program interval islikewise lower than the program voltage to inhibit disturbance of theswitching cell.

For a NAND string embodiment, a first switch (ground select switch orbottom bit line select switch) is provided on a first end of theplurality of transistors, and a second switch (string select switch ortop bit line select switch) is provided on a second end of the pluralityof transistors. In this embodiment, the control circuitry operates toturn off the first switch during the program interval to enable boostingby isolating the semiconductor body on the first side of the selectedword line. The control circuitry operates to turn on the second switchduring the program interval to connect the semiconductor body on asecond side of the selected word line to a corresponding bit line orreference line from which a reference voltage can be applied.

In a second plurality of memory cells coupled to the same plurality ofword lines, such as a parallel NAND string on an unselected bit line,the control circuitry can operate using a “boosted source” arrangement,by turning off the first switch for the second plurality of cells andturning off the second switch while applying pass voltages to memorycells on both the first and second side of the selected cell. In thisarrangement, the semiconductor body regions on both sides of theselected word line are boosted to similar voltage levels to prevent hotcarrier injection in unselected strings. Alternatively, the controlcircuitry can operate using a “grounded drain” arrangement, by turningon the first switch and turning on the second switch to bias asemiconductor region on both sides of the selected word line to areference voltage level to prevent hot carrier injection in unselectedstrings.

The control circuitry can operate to maximize the capacitance of thefirst semiconductor body region which is boosted to a boosted voltagelevel by applying a number of techniques. According to one technique,the plurality of memory cells can be extended to include one or moreextra memory cells along with one or more dummy word lines, and disposedbetween the plurality of memory cells and the first switch. In thistechnique, the control circuitry applies a pass voltage to theadditional dummy word line or word lines to expand the size, and therebythe capacitance, of the first semiconductor body region. According toanother technique, the control circuitry can organize the plurality ofword lines into a first set of word lines near one end of the pluralityof memory cells, and a second set of word lines near another end of theplurality of memory cells. To program a selected memory cell, thecontrol circuitry determines the one of the first and second sets ofwhich the selected word line is a member, and assigns the first side ofthe selected word line, which is to be boosted to the boosted voltagelevel, as that side including the other of the first and second sets. Inthis way, at least all of the word lines in one of the first and secondsets are used to establish the size of the first semiconductor bodyregion. In this manner, the first semiconductor body region used toestablish the boosted voltage level can be larger than the secondsemiconductor body region used to establish the reference voltage levelfor all of the memory cells in the string.

A method for inducing hot carrier injection for programming a selectedcell in a NAND string is described as based on blocking flow of carriersbetween a first semiconductor body region on a first side of theselected cell in the NAND string and a second semiconductor body regionon a second side of the selected cell well boosting the firstsemiconductor body region by capacitive coupling to a boosted voltagelevel and then biasing the second semiconductor body region to areference voltage level. A program potential higher than a hot carrierinjection barrier level is applied to the selected cell, and then theflow of carriers is enabled from the second semiconductor body region atthe reference voltage level to the selected cell. In this manner, theheating potential for hot carrier injection is provided by thecapacitively boosted virtual drain on the first side of the selectedcell.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description in the claims whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are simplified cross-sectional views of a selected NANDstring and an unselected NAND string biased according to prior artprogramming techniques.

FIGS. 2A and 2B are simplified cross-sectional views of a selected NANDstring showing first and second stages respectively of a programinterval for boosted node hot carrier injection.

FIG. 3 is a timing diagram for a program interval such as thatillustrated with respect to FIGS. 2A and 2B for a target memory cell.

FIG. 4 is a timing diagram for a program interval for a memory cellsharing a word line with the target memory cell in a parallel,unselected NAND string.

FIGS. 5A and 5B are simplified cross-sectional views of an unselectedNAND string showing first and second stages respectively of a programinterval for boosted node hot carrier injection showing how disturbanceof unselected cells prevented.

FIG. 6 is a schematic diagram of a common source type NAND-type memoryarray employing a program bias operation as described herein.

FIG. 7 is a schematic diagram of a common source type NAND-type memoryarray employing a program bias operation as described herein accordingto an alternative configuration.

FIG. 8 is a schematic diagram of a virtual ground NAND-type memory arrayemploying a program bias operation as described herein.

FIG. 9 is a schematic diagram of a virtual ground NAND-type memory arrayemploying a program bias operation as described herein according to analternative configuration.

FIG. 10 is a schematic diagram of a common source type NAND-type memoryarray employing a program bias operation as described herein accordingto an alternative configuration including more than one switching cell.

FIG. 11 is a simplified cross-sectional view of a selected NAND stringshowing a first stage of a program interval for boosted node hot carrierinjection, where the target cell is close to one end of the NAND string.

FIG. 12 is a simplified cross-sectional views of a selected NAND stringshowing a first stage of a program interval for boosted node hot carrierinjection, where the NAND string is extended by dummy word lines.

FIG. 13 is a simplified layout view of a NAND array showing aconfiguration for dummy word lines on the common source end of thestrings.

FIG. 14 is a simplified layout view of a NAND array showing aconfiguration for dummy word lines on the string select line end of thestrings.

FIG. 15 is a simplified layout view of a NAND array showing a logicalconfiguration word lines in the first and second sets operable so thatthe virtual drain side is always larger than the virtual source side ofthe selected memory cell.

FIG. 16 is a simplified layout view of a NAND array showing aconfiguration for dummy word lines on both ends of the strings.

FIG. 17 is a timing diagram for an alternative operating mode forinducing boosted node hot carrier injection as described herein.

FIG. 18 is a timing diagram for another alternative operating mode forinducing boosted node hot carrier injection as described herein.

FIG. 19 is a block diagram of an integrated circuit memory employingmemory cells and bias circuitry according to embodiments of the presentinvention.

DETAILED DESCRIPTION

A detailed description of embodiments of the present invention isprovided with reference to the FIGS. 1-19.

FIGS. 1A and 1B show in cross-section a plurality of dielectric chargetrapping flash memory cells arranged in series to form a NAND string andbiased for FN tunneling programming, as is typical in prior art NANDarchitecture flash memory. FIG. 1A shows the biasing for a NAND stringthat includes a target cell on a selected bit line, while FIG. 1B showsthe biasing for a NAND string on an unselected bit line. One technologyfor implementation of NAND flash using bandgap engineered SONOS(BE-SONOS) charge trapping technology is described in U.S. Pat. No.7,315,474 by Lue, which is incorporated by reference as if fully setforth herein. NAND strings can be implemented in a variety ofconfigurations, including finFET technology, shallow trench isolationtechnology, vertical NAND technology and others. See, for an examplevertical NAND structures, European Patent Application No. EP 2 048 709by Kim et al. entitled “Non-volatile memory device, method of operatingsame and method of fabricating the same.”

Referring to FIG. 1A, the memory cells are formed in a semiconductorbody 10. For n-channel memory cells, the semiconductor body 10 can be anisolated p-well, within a deeper n-well in a semiconductor chip.Alternatively, the semiconductor body 10 can be isolated by aninsulating layer or otherwise. Some embodiments may employ p-channelmemory cells in which the doping for the semiconductor body would ben-type.

The plurality of flash memory cells is arranged in a string extending ina bit line direction, orthogonal to word lines. Word lines 22-27 extendacross a number of parallel NAND strings. Terminals 12-18 are formed byn-type regions (for n-channel devices) in the semiconductor body 10, andact as the source/drain regions for the memory cells. A first switchformed by a MOS transistor having a gate in a ground select line GSL 21is connected between the memory cell corresponding with first word line22 and a contact 11 formed by an n-type region in the semiconductor body10. The contact 11 is connected to common source CS line 30. A secondswitch formed by a MOS transistor having a gate in a string select lineSSL 28 is connected between the memory cell corresponding to the lastword line 27 and a contact 19 formed by an n-type region in thesemiconductor body 10. The contact 19 is connected to a bit line BL 31.The first and second switches in the illustrated embodiment are MOStransistors, having gate dielectrics 7 and 8 formed by for examplesilicon dioxide.

In this illustration, there are six memory cells in the string forsimplicity. In typical implementations, a NAND string may comprise 16,32 or more memory cells arranged in series. The memory cellscorresponding to the word lines 22-27 have charge trapping structures 9between the word lines and channel regions in the semiconductor body 10.The charge trapping structures 9 in the memory cells can be dielectriccharge trapping structures, floating gate charge trapping structures, orother flash memory structures suitable for programming using techniquesdescribed herein. Also, embodiments of NAND flash structures have beendeveloped which are junction-free, where the terminals 13-17, andoptionally terminals 12 and 18, may be omitted from the structure.

FIG. 1A shows the biasing according to a typical prior art technology toinduce FN tunneling to program the memory cell corresponding with wordline 24. According to the biasing illustrated, the GSL is biased toabout zero volts while the common source line is grounded, so that thefirst switch corresponding to the GSL line 21 is off, and the SSL isbiased to about VCC while the selected bit line is grounded, so that thesecond switch corresponding to the SSL line 28 is on. In theseconditions, the semiconductor body in the region 33 associated with theNAND string is pre-charged to about 0 Volts. The selected word line 24is biased at a high-voltage programming level V-PGM, which can be on theorder of 20 volts in some embodiments. The unselected word lines 22, 23and 25-27 are biased to a pass voltage V-PASS, which is less than V-PMGby an amount that inhibits programming in unselected memory cells in thestring. As a result, electrons tunnel into the charge trapping structureof the selected memory cell.

FIG. 1B shows the biasing according to prior art technology atunselected bit lines for NAND strings which share the word lines 22-27with the string shown in FIG. 1A. As can be seen, the GSL, the SSL andall of the word lines have the same bias voltages as shown in FIG. 1A.Likewise, the common source line 30 is grounded. However, the unselectedbit line 32 is biased to a level about VCC. This turns off the secondswitch, which corresponds with the SSL line, and decouples thesemiconductor body in the region 35 from the unselected bit line BL 32.As a result, the semiconductor body in the region 35 is boosted bycapacitive coupling from the voltages applied to the word lines 22-27,which prevents formation of electric fields sufficient to disturb thecharge trapped in the memory cells in the unselected NAND string. Socalled incremental step pulsed programming ISPP operations based oncapacitive self-boosting are common in the art.

FIGS. 2A and 2B show two stages of a two-stage programming bias for aselected NAND string, for boosted-drain, hot carrier programming asdescribed herein. For n-channel embodiments, the hot carriers includeelectrons. For p-channel embodiments, similar bias techniques can beapplied to induce hot hole injection, in which the hot carriers includeholes. The program process is described with reference to the n-channelembodiment herein, but is referred to as boosted node hot carrierinjection in reference to alternative p-channel embodiments.

In FIG. 2A, a first stage is illustrated, in which the common source CSline 30 is grounded, and the selected bit line 31 is also coupled toabout zero volts. The GSL line 21 is coupled to about zero volts turningoff the first switch 42, decoupling the semiconductor body from the CSline 30. The SSL line 28 is coupled to about VCC, turning on the secondswitch 43, and coupling the semiconductor body to the selected bit line31. The word line corresponding to the target memory cell 40 receives aprogram pulse at V-PGM. The word line adjacent the target memory cell 40on the side of the bit line BL 31 receives a two stage switching voltageV-SW which, during this first stage of the program interval, is at a lowvoltage so that the channel of switching memory cell 41 is off, andserves to isolate the regions 50 and 51 in the semiconductor body. Underthe biasing condition during a program interval, the region 50 in thesemiconductor body 10 is boosted to a virtual drain voltage Vd bycapacitive coupling in response to the pass voltages V-PASS (drain side)on the word lines between the target word line which receives V-PGM andthe first switch 42. The region 51 in the semiconductor body 10 ispre-charged to a virtual source voltage Vs by coupling of the bit line31 biased to 0 Volts to the substrate. The voltage V-PASS (source side)is coupled to the word lines between the switching cell 41 and thesecond switch 43. V-PASS (source side) may be the same voltage as V-PASS(drain side), or may be different as suits a particular implementationor programming condition. The boosted voltage level in region 50 and thereference voltage level in the region 51 are isolated by the depletedregion 52 beneath the switching memory cell during this first stage ofthe program interval.

In this example, as in all example NAND strings shown herein, the firstand second switches (42, 43) are implemented by field effect transistorsin series with the memory cells in the string. In the example shown inFIG. 2A, the gate dielectrics for the field effect transistors are drawnas single layer structures, and typically comprise a layer of a siliconoxide or nitrogen doped silicon oxide. In other the implementations, thefield effect transistor used for the switches (e.g. 42, 43) in exampleNAND strings shown herein, can have a multilayer gate dielectric,including a gate dielectric that is implemented in the same manner asthe charge storage layers in the memory cells in the string. Thisapproach can simplify the manufacturing process. In such embodiments,the first and second switches can be characterized as being “memorycells.” The channel lengths of the field effect transistors used toimplement the switches can be longer than the channel length in memorycells if desired.

In FIG. 2B, the second stage of the program interval is illustrated,where the switching voltage V-SW changes to turn on the switching memorycell 41 adjacent the target memory cell 40. The difference between Vdand Vs during the transition is sufficient to induce hot carriers 54 inthe channel of the target memory cell. The voltage V-PGM on the wordline corresponding to the target memory cell is sufficient to overcomethe barrier height for the hot carriers, and as a result hot carrierinjection programming is induced. A program operation can include asequence of program intervals as described with reference to FIGS. 2Aand 2B, with intervening program verify steps, to efficiently achievetarget threshold values. Embodiments can apply this technology formultilevel programming to store more than one bit per cell.

FIG. 3 is a timing diagram showing the voltage waveform for the twostage program interval shown in FIGS. 2A-2B for a selected bit line.During a bit line set up interval, the SSL line is increased to a levelabout VCC. During the set up interval, the voltage level Vd of thevirtual drain region 50 and the voltage level Vs of the virtual sourceregion 51 remain at about zero volts. During a program interval, thevoltage V-PGM is pulsed to a level sufficient to overcome the hotcarrier injection barrier level as discussed above. Also, in the firststage of the program interval, which can be referred to as a VDS setupstage, voltage V-PASS is pulsed to a level less than V-PGM by an amountsufficient to inhibit programming of other cells in the string. Thevoltage V-PASS may be lower on the virtual source side than on thevirtual drain side in some implementations. During the first stage ofthe program interval, the voltage V-SW remains at a low voltage to turnoff the cell 41. The virtual drain region 50 is boosted by capacitivecoupling so that virtual drain voltage Vd rises above the VCC level inthis example, while the virtual source voltage Vs remains at about zerovolts. After a sufficient interval to allow the drain to source voltageVDS for the target memory cell to reach a level at which hot carrierinjection can be induced, a second stage of the program interval isbegun, which can be referred to as the program stage. In the secondstage of the program interval, the voltage V-SW is pulsed to a switchingvoltage, which has a level in this embodiment which is not higher thanV-PASS. During at least a first part of the interval represented byshaded region 90, while the drain/source voltage VDS remains sufficientto induce hot carriers, hot carrier injection occurs to program thetarget memory cell. After V-PASS and V-PGM fall at the end of theprogram stage of the program interval, the SSL line may remain at VCCfor an interval in which the semiconductor body can be dischargedthrough the selected bit line.

FIG. 4 shows the voltage waveform for the program interval for a NANDstring on an unselected bit line, which shares word lines with theselected NAND string. In the NAND string on the unselected bit line, thesemiconductor body voltage level rises during the bit line set upinterval to the first level, and is boosted by the word line voltagesduring the first and second stages of the program interval so that thevirtual drain and virtual source voltages are equal, or close to equal,when the second stage of the program interval begins. As a result, hotcarriers are not generated in NAND strings on the unselected bit lines,and the memory cells are not disturbed.

FIGS. 5A and 5B show two stages of a programming bias for an unselectedNAND string, in the event that a NAND string sharing word lines isselected for boosted-node hot carrier programming as described herein,under the bias condition shown in FIG. 4. In FIG. 5A, a first stage isillustrated, in which the common source CS line 30 is grounded, and theunselected bit line 32 is coupled to about VCC, rather than to 0 V as isapplied to the selected bit line. The GSL line 21 is coupled to aboutzero volts turning off the first switch 42 u, decoupling thesemiconductor body from the CS line 30. The SSL line 28 is coupled toabout VCC, which does not turn on the second switch 43 u, therebydecoupling the semiconductor body from the unselected bit line 32. Theword line corresponding to the unselected target memory cell 40 ureceives a program pulse at V-PGM. The word line adjacent the unselectedtarget memory cell 40 u on the side of the bit line receives a switchingvoltage V-SW, which during this first stage of the program interval isat a low voltage so that the switching memory cell 41 u serves toisolate the regions 50 and 60 in the semiconductor body. Under thebiasing condition during the first stage of the program interval, theregion 50 in the semiconductor body 10 is boosted to a virtual drainvoltage Vd by capacitive coupling in response to the pass voltagesV-PASS (drain side) on the word lines between the target word line whichreceives V-PGM and the first switch 42 u. The region 60 in thesemiconductor body 10 on the unselected bit line is also boosted andachieves a level Vs that is close to Vd, by capacitive coupling inresponse to the pass voltages V-PASS (source side). The boosted voltagelevel in region 50 and the reference voltage level in the region 60 areclose to one another, but remain isolated by the depleted region 61beneath the switching memory cell 41 u.

In FIG. 5B, the second stage of the program interval is illustrated,where the switching voltage V-SW changes to turn on the switching memorycell 41 u, coupling the regions 50 and 60 together to form boostedregion 62. The difference between Vd and Vs during the transition iszero, or is a level that is too low to induce hot carriers in thechannel of the memory cell corresponding to the target word line. Thevoltage V-PGM on the word line corresponding to the unselected targetmemory cell 40 u is insufficient to induce FN tunneling in the region63, and so the unselected memory cell 40 on the unselected bit line isnot disturbed.

Representative bias levels for program and erase operations are shown inthe following table.

Program Erase Unselected WL 6 V~12 V −8 V Selected WL 10 V~16 V  −8 VSwitch WL (in 4 V~12 V −8 V second stage) Unselected BL VCC FloatingSelected BL 0 V Floating PW 0 V 12 V SSL VCC Floating/VCC GSL 0 VFloating/VCC Common Source 0 V Floating

FIG. 6 is a circuit diagram showing layout of four NAND strings 101,102, 103, 104 coupled to respective bit lines BL-1 to BL-4 and to acommon source CS line 105 by string select transistors (e.g. 112) andground select transistors (e.g. 111), respectively. For the purpose ofillustration, the bias voltages are shown for programming a targetmemory cell 100 on corresponding word line WL(i) in the NAND string 101.The first switch transistor 111 is biased by ground on the GSL line todecouple the NAND string from the CS line 105. The second switchtransistor 112 is biased by the SSL line to couple the NAND string tothe selected bit line BL-1. A switching memory cell 113 on acorresponding word line WL(i−1) is adjacent the target memory cell 100.Thus, the word line WL(i−1) receives V-SW supporting the two-stageprogram interval. During the first stage of the program interval, thesemiconductor body in region 120 is biased to the virtual source voltageVs about zero volts, and the semiconductor body in the region 121 isbiased by capacitive coupling to the virtual drain voltage Vd. On theunselected bit lines, the regions 122, 123 are biased by capacitivecoupling also to a relatively high voltage. Thus, when the second stageof the program interval starts, hot carrier injection happens in thetarget memory cell 100, while the other memory cells in the array arenot disturbed. Note that when a target memory cell is on the first wordline WL(0), the string select line SSL can be used to apply theswitching voltage V-SW to the switch transistor 112, allowing the bitline terminal for the NAND string to operate as the virtual source.

FIG. 7 shows the bias conditions in which the switching memory cell 113is adjacent to target memory cell 100 on the common source CS side ofthe string. Thus, FIG. 7 is a circuit diagram showing layout of fourNAND strings 101, 102, 103, 104 which are coupled to respective bitlines BL-1 to BL-4 and to a common source CS line 105 by string selecttransistors and ground select transistors, respectively. The biasvoltages are shown for programming a target memory cell 100 on acorresponding word line WL(i) in the NAND string 101. The first switchtransistor 111 is biased by VCC on the GSL line to couple the NANDstring to the grounded CS line 105. The second switch transistor 112 isbiased by VCC on the SSL line and VCC on the selected bit line BL-1 todecouple the NAND string from the selected bit line BL-1. A switchingmemory cell 113 on corresponding word line WL(i+1) is adjacent thetarget memory cell 100. Thus, the word line WL(i+1) receives V-SWsupporting the two stage program interval. During the first stage of theprogram interval, the semiconductor body in region 150 is biased to thevirtual drain voltage Vd by capacitive coupling. The semiconductor bodyin the region 151 is biased to the virtual source voltage Vs via the CSline. On the unselected bit lines, which are coupled to 0 V, the region152 is biased to ground via unselected bit lines BL-2 to BL-4 and theregion 153 is biased to ground via the CS line 105. Thus, when thesecond stage of the program interval starts, hot carrier injectionhappens in the target memory cell 100, while the other memory cells inthe array are not disturbed.

FIGS. 6 and 7 show the possibility of using biasing in two directions,from the top and from the bottom of the string, within a single arrayconfiguration. This can be beneficial in order to ensure that the partof the semiconductor body acting as a virtual drain has sufficientcapacitance for maintaining hot carrier injection current needed forreasonable programming speed. For example, the programming controllercan be implemented to bias the array so that the virtual drain side ofthe target memory cell has at least one half of the total number of wordlines within the NAND string.

FIG. 8 is a circuit diagram showing layout of seven NAND strings 201-207arranged in a virtual ground NAND architecture. In a virtual groundarchitecture as described here, the bit lines act as both bit linescoupled to sense amplifiers and reference lines coupled to referencevoltage sources, depending on the column being accessed. The NANDstrings are coupled to corresponding pairs of bit lines BL-1 to BL-8 bytop bit line select transistors BLT and bottom bit line selecttransistors BLB. For the purpose of illustration, the bias voltages areshown for programming a target memory cell 300 on corresponding wordline WL(i) in the NAND string 204. The first switch transistor 301 isbiased by VCC on the BLB line to couple the NAND string 204 to BL-5,which is grounded. The second switch transistor 302 is biased by VCC onthe BLT line to decouple the NAND string 204 from the bit line BL-4,which is biased at VCC. All of the bit lines BL-1 to BL-3 to the left oftarget NAND string 204, are biased at VCC. All of the bit lines BL-6 toBL-8 to the right of the target NAND string 204 are biased at ground. Aswitching memory cell 304 on corresponding word line WL(i+1) is adjacentthe target memory cell 300. Thus, the word line WL(i+1) receives theV-SW supporting the two stage program interval. During the first stageof the program interval, the semiconductor body in region 311 is biasedto the virtual source voltage Vs about zero volts. The semiconductorbody in the region 310 is biased by capacitive coupling to the virtualdrain voltage Vd, thereby setting up for the second stage of the programinterval in which hot carrier injection causes programming of the targetcell 300. On the unselected strings to the right, the regions 312 and313 are biased to ground potential via the bit lines to BL-5 to BL-8 toavoid disturbance of memory cells in those strings. On the unselectedstrings to the left, the regions 314 and 315 are boosted by capacitivecoupling to a relatively high voltage to avoid disturbance of memorycells in those strings. Thus, when the second stage of the programinterval starts, hot carrier injection happens in the target memory cell300, while the other memory cells in the array are not disturbed.

FIG. 9 shows a biasing arrangement for a virtual ground NAND array likethat shown in FIG. 8, in which the switching transistor is on the otherside. The NAND strings are coupled to corresponding pairs of bit linesBL-1 to BL-8 by top bit line select transistors BLT and bottom bit lineselect transistors BLB. For the purpose of illustration, the biasvoltages are shown for programming a target memory cell 320 oncorresponding word line WL(i+1) in the NAND string 204. The first switchtransistor 321 is biased by VCC on the BLB line, which decouples theNAND string 204 from BL-5, which is biased at VCC. The second switchtransistor 322 is biased by VCC on the BLT line to couple the NANDstring to the bit line BL-4 which is grounded. All of the bit lines BL-1to BL-3 to the left of NAND string 204 are grounded. All of the bitlines BL-6 to BL-8 to the right of NAND string 204 are biased at VCC. Aswitching memory cell 324 on corresponding word line WL(i) is adjacentthe target memory cell 320. Thus, the word line WL(i) receives V-SWsupporting the two-stage program interval. During the first stage of theprogram interval, the semiconductor body in region 330 is biased to thevirtual source voltage Vs about zero volts. The semiconductor body inthe region 331 is biased by capacitive coupling to the virtual drainvoltage Vd, thereby setting up for the second stage of the programinterval in which hot carrier injection causes programming of the targetcell 320. On the unselected strings to the right, the regions 332 and333 are boosted by capacitive coupling also to a relatively high voltageto avoid disturbance of memory cells in those strings. On the unselectedstrings to the left, the regions 334 and 335 are biased to groundpotential via the bit lines BL-1 to BL-4 to avoid disturbance of memorycells in those strings. Thus, when the second stage of the programinterval starts, hot carrier injection happens in the target memory cell320, while the other memory cells in the array are not disturbed.

FIG. 10 shows the bias conditions for a NAND array like that of FIGS. 6and 7, in which two switching memory cells 155, 156 are adjacent totarget memory cell 157 on the common source CS side of the string. FIG.10 is a circuit diagram showing layout of four NAND strings 101, 102,103, 104 which are coupled to respective bit lines BL-1 to BL-4 and to acommon source CS line 105 by string select transistors and ground selecttransistors respectively. The bias voltages are shown for programming atarget memory cell 157 on corresponding word line WL(i+1) in the NANDstring 101. The first switch transistor 111 is biased by ground on theGSL line to decouple the NAND string from the grounded CS line 105. Thesecond switch transistor 112 is biased by VCC on the SSL line to couplethe NAND string to the selected bit line BL-1, which is biased toground. A switching memory cell 155 on corresponding word line WL(i−1)and a switching memory cell 156 on corresponding word line WL(i) areadjacent the target memory cell 157. Thus, the word lines WL(i−1) andWL(i) receive respective V-SW signals in support of the two stageprogram interval, which can be the same or somewhat different as suits aparticular implementation. During the first stage of the programinterval, the semiconductor body in region 160 is biased to the virtualsource voltage Vs about zero volts. The semiconductor body in the region161 is biased by capacitive coupling to the virtual drain voltage Vd. Onthe unselected bit lines which are biased to VCC, thereby decoupling thecorresponding NAND strings from the bit lines, the regions 162, 163 arebiased by capacitive coupling also to a relatively high voltage. Thus,when the second stage of the program interval starts, hot carrierinjection happens in the target memory cell 157, while the other memorycells in the array are not disturbed. The use of two switching memorycells 155, 156 to isolate the virtual drain region 161 and the virtualsource region 160 during the first stage of the program interval cansuppress leakage current, including sub-threshold leakage, during theset up stage of the program interval.

FIG. 11 is a cross-section of a NAND string like that of FIGS. 2A and2B. FIG. 11 shows biasing for the first stage in the condition that thetarget memory cell 180 is close to one end of the string, such as closeto the ground select line GSL. In this condition, during the first stageof the program interval the common source CS line 30 is grounded, andthe selected bit line 31 is also coupled to about zero volts. The GSLline 21 is coupled to about zero volts turning off the first switch 42,decoupling the semiconductor body from the CS line 30. The SSL line 28is coupled to about VCC, turning on the second switch 43, and couplingthe semiconductor body to the selected bit line 31. The word linecorresponding to the target memory cell 180 receives a program pulse atV-PGM. The word line adjacent the target memory cell 180 on the side ofthe bit line receives a switching voltage V-SW establishing theswitching memory cell 181. During the first stage of the programinterval, V-SW is at a low-voltage so that the switching memory cell 181serves to isolate the region of 183 and 184 in the semiconductor body.Under the biasing condition during a program setup interval, the region184 in the semiconductor body 10 is boosted to a virtual drain voltageVd by capacitive coupling in response to the pass voltage V-PASS (drainside) on the word line for the cell between the target word line whichreceives V-PGM and the GSL line. The region 183 in the semiconductorbody 10 is pre-charged to a virtual source voltage Vs by coupling of thebit line 31 to the substrate. The voltage V-PASS (source side) iscoupled to the word lines between the switching word line at cell 181and the second switch 43. The boosted voltage level in region 184 andthe reference voltage level in the region 183 are isolated by thedepleted region beneath the switching memory cell 181. However, in thiscondition, the virtual drain region 184 is small, and the therefore willhave a relatively small capacitance. Small capacitance will cause theamount of hot carrier generation during the region 90 of FIG. 3 to besmaller, and reduce the amount of hot carrier injection that could beachieved in a single program interval.

Thus, as shown in FIG. 12, an alternative embodiment utilizes one ormore dummy word lines (401, 402) between the GSL and the plurality ofmemory cells in the NAND string to improve the minimum programmingefficiency. FIG. 12 is a cross-section of a NAND string like that ofFIG. 11. FIG. 12 shows biasing for the first stage in the condition thatthe target memory cell 480 is close to one end of the string, such asclose to the ground select line GSL. In this condition, during the firststage of the program interval the common source CS line 30 is grounded,and the selected bit line 31 is also coupled to about zero volts. TheGSL line is coupled to about zero volts turning off the first switch 42,decoupling the semiconductor body from the CS line 30. The SSL line iscoupled to about VCC, turning on the second switch 43, and coupling thesemiconductor body to the selected bit line 31. The word linecorresponding to the target memory cell 480 receives a program pulse atV-PGM. The word line adjacent the target memory cell 480 on the side ofthe bit line receives a switching voltage V-SW establishing the memorycell 481 as the switching memory cell. During the first stage of theprogram interval V-SW is at a low-voltage so that the switching memorycell 481 serves to isolate the regions 483 and 484 in the semiconductorbody. Under the biasing condition during the first stage programinterval, the region 484 in the semiconductor body 10 is boosted to avirtual drain voltage Vd by capacitive coupling in response to the passvoltage V-PASS (drain side) on the word line 482, and on the dummy wordlines 401 and 402, between the target word line which receives V-PGM andthe GSL line. The region 483 in the semiconductor body 10 is pre-chargedto a virtual source voltage Vs by coupling of the bit line 31 to thesubstrate. The voltage V-PASS (source side) is coupled to the word linesbetween the switching word line at cell 481 and the second switch 43.V-PASS (source side) may be the same voltage as V-PASS (drain side), ormay be different as suits a particular implementation or programmingcondition. The boosted voltage level in region 484 and the referencevoltage level in the region 483 are isolated by the depleted regionbeneath the switching memory cell 181. As shown, in this condition, thevirtual drain region 484 is guaranteed to include at least two cellsunder the dummy word lines 401 in 402, and the therefore will have acapacitance that enables a greater amount of hot carrier injectionduring the program interval. Note that the dummy cells can be used asthe switching cells for programming the memory cell corresponding withword line 482 as a mode applying the CS line side as the virtual source.

FIG. 13 shows a simplified layout view showing the word lines andsource-drain strings of a NAND array like that of FIG. 12 with dummyword lines DWL1 and DWL2 adjacent the GSL line. Thus, source-drainstrings 500-503 extend vertically on the page. Horizontal conductorlines overlie the source-drain strings 500-503. The horizontal conductorlines include the SSL line, word lines WL0 to WL(N−1) and the dummy wordlines DWL1 and DWL2. Also, the horizontal conductor lines include theGSL line and the common source CS conductor.

FIG. 14 shows a simplified layout view showing the word lines andsource-drain strings of a NAND array like that of FIG. 12 with dummyword lines on the opposite end of the array, adjacent the SSL line.Thus, source-drain strings 500-503 extend vertically on the page.Horizontal conductor lines overlie the source-drain strings 500-503. Thehorizontal conductor lines include the SSL line, the dummy word linesDWL1 and DWL2, and the word lines WL0 to WL(n−1). Also, the horizontalconductor lines include the GSL line and the common source CS conductor.

FIG. 15 shows a simplified layout view showing the word lines andsource-drain strings of a NAND array like that of FIG. 12 without dummyword lines. However, the word lines are logically arranged into a set oftop word lines TWL(0) to TWL(N−1) (only TWL(0) to TWL(4) are shown) anda set of bottom word lines BWL(0) to BWL(M−1) (only BWL(M−5) to BWL(M−1)are shown). Thus, when a target cell falls within the top word lines,the programming operation is arranged so that the virtual drain regionincludes all the semiconductor body region beneath the bottom wordlines. When the target cell falls within the bottom word lines, theprogramming operation is arranged so the virtual drain region includesall of the semiconductor body region beneath top word lines. In thismanner, the programming performance for the hot carrier injection isimproved.

FIG. 16 shows a simplified layout view showing the word lines andsource-drain strings of a NAND array like that of FIG. 12 with dummyword lines adjacent the GSL line and dummy word lines adjacent the SSLline. Thus, source-drain strings 500-503 extend vertically on the page.Horizontal conductor lines overlie the source-drain strings 500-503. Thehorizontal conductor lines include the SSL line, top dummy word linesTDWL1 and TDWL2, word lines WL0 to WL(N−1) and the bottom dummy wordlines BDWL1 and BDWL2. Also, the horizontal conductor lines include theGSL line in the common source CS conductor.

FIGS. 17 and 18 illustrate alternative timing arrangements for theprogram interval used to induce boosted-node hot carrier injection asdescribed herein. These sequences include turning on the second switchby a high level on SSL during at least a part of the first stage of theprogram interval when switching voltage V-SW is low, and turning off thesecond switch by transitioning to a low level on SSL during at least apart of the second stage of the program interval in which the switchingvoltage V-SW is high. As shown in FIG. 17, during a program interval,the selected bit line BL, the ground select line GSL and the commonsource line CS are kept at ground potential while the unselected bitline BL is biased to VCC. At the beginning of the program interval attime 600, the SSL is biased to VCC, coupling ground to the semiconductorbody. A short time after SSL switches to VCC at time 601, the targetword line receives the V-PGM potential, the adjacent word line for theswitching cell receives V-SW which is low enough to turn off theswitching cell, and the other word lines along the NAND string receivethe V-PASS potential. This sets the virtual drain region and virtualsource region voltages as shown in FIG. 2A. According to the process ofFIG. 17, SSL switches back to ground potential at time 602, rather thancontinuing at VCC during the entire program interval as shown in FIG. 3.The voltage V-SW switches to the higher voltage level at time 603, whichcan coincide with time 602. The program interval is over at time 604when the program potential returns to ground, along with the othersignals.

As shown in FIG. 18, a delay interval 606 can be introduced between thetime 602 that the SSL line goes to ground and the time 605 that V-SWgoes high. As before, during a program interval, the selected bit line,the ground select line and the common source line are kept at groundpotential while the unselected bit line is biased to VCC. At thebeginning of the program interval at time 600, the SSL is biased to VCC,coupling ground to the semiconductor body. A short time after SSLswitches to VCC at time 601, the target word line receives the V-PGMpotential, and the other word lines along the NAND string receive theV-PASS potential. In this sequence, V-SW transitions to the high levelat time 605, after a delay of 606 from the time that SSL goes low. Theprogram interval is over at time 604 when the program potential returnsto ground, along with the other signals. These processes that turn offboth the GSL and SSL switches can operate at lower power.

FIG. 19 is a simplified block diagram of an integrated circuit employingboosted virtual drain, hot carrier injection programmed NAND flash asdescribed herein. The integrated circuit 810 includes a memory array 812implemented using charge trapping memory cells or floating gate memorycells, for example, on a semiconductor substrate. A word line (or row)and string select decoder 814 (including appropriate drivers) arecoupled to, and in electrical communication with, a plurality 816 ofword lines, string select lines and ground select lines, arranged alongrows in the memory array 812. A bit line (column) decoder and drivers818 are coupled to and in electrical communication with a plurality ofbit lines 820 arranged along columns in the memory array 812 for readingdata from, and writing data to, the memory cells in the memory array812. Addresses are supplied on bus 822 to the word line decoder andstring select decoder 814 and to the bit line decoder 818. Senseamplifiers and data-in structures in block 824, including currentsources for the read, program and erase modes, are coupled to the bitline decoder 818 via data bus 826. Data is supplied via the data-in line828 from input/output ports on the integrated circuit 810 or from otherdata sources internal or external to the integrated circuit 810, to thedata-in structures in block 824. In the illustrated embodiment, othercircuitry 830 is included on the integrated circuit 810, such as ageneral purpose processor or special purpose application circuitry, or acombination of modules providing system-on-a-chip functionalitysupported by the memory cell array. Data is supplied via the data-outline 832 from the sense amplifiers in block 824 to input/output ports onthe integrated circuit 810, or to other data destinations internal orexternal to the integrated circuit 810.

A controller 834 implemented in this example, using bias arrangementstate machine, controls the application of bias arrangement supplyvoltages and current sources 836, such as read, program, erase, eraseverify, program verify voltages or currents for the word lines and bitlines, and controls the word line/source line operation using an accesscontrol process. The controller implements the switching sequences usedto induce boosted-node hot carrier programming as described herein. Thecontroller 834 can be implemented using special purpose logic circuitryas known in the art. In alternative embodiments, the controller 834comprises a general purpose processor, which may be implemented on thesame integrated circuit, which executes a computer program to controlthe operations of the device. In yet other embodiments, a combination ofspecial-purpose logic circuitry and a general-purpose processor may beutilized for implementation of the controller 834.

A new program method for NAND flash is provided which suppresses programdisturb due to lower operation voltage. A new program based on use ofboosted node potentials to achieve hot carrier injection enables use ofreduced operation voltage. As a result of reduced operation voltages,the driving circuitry on the integrated circuit can be implemented usingonly one MOSFET process, without requiring additional high voltageMOSFET processes.

Compared to conventional channel hot electron injection operation, theBL voltage does not need to overcome the hot carrier injection barrierheight. Thus, the BL voltage can be VCC or other voltages that are lowerthan requirements for conventional CHE programming voltage. Furthermore,the BL will not consume DC current during the hot carrier injection. So,the new program operation should achieve low power consumption.

Also, the WL voltage of this program method is lower than that requiredfor conventional NAND flash FN programming operation. Thus very highvoltage driving devices are not needed. Also, the vertical electricfield across the tunnel oxide in the NAND flash array is smaller thanthat required for FN injection. As a result of lower E-fieldrequirements, device reliability is better.

Furthermore, lower program and Vpass voltages than required forconventional FN operation cause reduced inter-WL dielectric voltages,and thus mitigate inter-WL dielectric breakdown issues that arise as thespacing between word lines shrinks.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

What is claimed is:
 1. A memory comprising: a NAND string including aplurality of memory cells arranged in series in a semiconductor body; aplurality of word lines, word lines in the plurality coupled tocorresponding memory cells in the plurality of memory cells; and controlcircuitry coupled to the plurality of word lines adapted for programminga selected memory cell in the plurality of memory cells corresponding toa selected word line using a bias arrangement having a first stage and asecond stage, the biasing arrangement including: during the first stage,blocking flow of carriers between a first semiconductor body region on afirst side of a selected cell in the NAND string and a secondsemiconductor body region on a second side of the selected cell in theNAND string; during the first stage, isolating the first semiconductorbody region and applying a pass voltage pulse on word lines in theplurality of word lines on the first side of the selected cell to boostby capacitive coupling the first semiconductor body region to a boostedvoltage level; during the first stage and the second stage, biasing thesecond semiconductor body region to a reference voltage level; duringthe first stage and the second stage, applying a program potentialgreater than a hot carrier injection barrier level to the selected cell;and during the second stage, enabling flow of carriers from the secondsemiconductor body region to the selected cell to cause generation ofhot carriers.
 2. The device of claim 1, said programming includingapplying a two stage switching voltage to a cell in the NAND stringadjacent the selected cell, including during the first stage turning offthe cell to cause said blocking, and during the second stage turning onthe cell to cause said enabling.
 3. The device of claim 1, wherein NANDstrings in the NAND array include a first switch between a first end ofthe NAND string and a bit line or reference line and a second switchbetween a second end of the NAND string and a bit line or referenceline, and wherein said boosting includes: during the first stage,isolating the first semiconductor body region by turning off the firstswitch in a NAND string including the selected cell to isolate the firstsemiconductor body region, while turning on the second switch andapplying a reference voltage to the second semiconductor body region viathe second switch.
 4. The device of claim 3, including turning off thefirst and second switches in unselected NAND strings during saidboosting.
 5. The device of claim 3, including turning on the first andsecond switches in unselected NAND strings during said boosting.
 6. Thedevice of claim 1, wherein NAND strings in the NAND array include afirst switch between a first end of the NAND string and a bit line orreference line and a second switch between a second end of the NANDstring and a bit line or reference line, and wherein the NAND strings inthe array each include a first group of M cells proximal to the firstswitch, and a second group of N cells proximal to the second switch, andif the selected cell is in the first group of M cells, then biasing theNAND string so that the first semiconductor body region includes atleast a second group of N cells, and if the selected cell is in thesecond group of N cells, then biasing the NAND string so that the firstsemiconductor body region includes at least a first group of M cells. 7.A method for inducing hot carrier injection in a selected cell in NANDstring in a NAND array, comprising: using a bias arrangement having afirst stage and a second stage, the biasing arrangement including duringthe first stage, blocking flow of carriers between a first semiconductorbody region on a first side of a selected cell in the NAND string and asecond semiconductor body region on a second side of the selected cellin the NAND string; during the first stage, isolating the firstsemiconductor body region and applying a pass voltage pulse on wordlines in the plurality of word lines on the first side of the selectedcell to boost by capacitive coupling the first semiconductor body regionto a boosted voltage level; during the first stage and the second stage,biasing the second semiconductor body region to a reference voltagelevel; during the first stage and the second stage, applying a programpotential greater than a hot carrier injection barrier level to theselected cell; and during the second stage, enabling flow of carriersfrom the second semiconductor body region to the selected cell to causegeneration of hot carriers.
 8. The method of claim 7, including applyinga two stage switching voltage to a cell in the NAND string adjacent theselected cell, including during the first stage turning off the cell tocause said blocking, and during the second stage turning on the cell tocause said enabling.
 9. The method of claim 7, wherein NAND strings inthe NAND array include a first switch between a first end of the NANDstring and a bit line or reference line and a second switch between asecond end of the NAND string and a bit line or reference line, andwherein said boosting includes: during the first stage, isolating thefirst semiconductor body region by turning off the first switch in aNAND string including the selected cell to isolate the firstsemiconductor body region, while turning on the second switch andapplying a reference voltage to the second semiconductor body region viathe second switch.
 10. The method of claim 9, including turning off thefirst and second switches in unselected NAND strings during saidboosting.
 11. The method of claim 9, including turning on the first andsecond switches in unselected NAND strings during said boosting.
 12. Themethod of claim 7, wherein NAND strings in the NAND array include afirst switch between a first end of the NAND string and a bit line orreference line and a second switch between a second end of the NANDstring and a bit line or reference line, and wherein the NAND strings inthe array each include a first group of M cells proximal to the firstswitch, and a second group of N cells proximal to the second switch, andif the selected cell is in the first group of M cells, then biasing theNAND string so that the first semiconductor body region includes atleast a second group of N cells, and if the selected cell is in thesecond group of N cells, then biasing the NAND string so that the firstsemiconductor body region includes at least a first group of M cells.